Only one thing can really hurt him like that, and finding out someone has turned to the Dark is neither likely nor sufficient, in my opinion. But I also have a feeling Cadsuane will die. She's the only one of the women close to him that is expendable. In any case, there will be a confrontation with Aran'gar and/or Graendal, and Aran'gar seems the most likely for Rand to be tricked into killing. Out of all the women in his litany, he's only killed a few directly: the nameless Darkfriend woman in The Dragon Reborn, Liah (mercy killing - Mashadar had her, so he balefired her before her pain began), and Ilyena (whom he only killed because the Dark One tainted saidin). Anyway, it's about time he killed another one directly...